The reaction of thrombin with platelets will be examined by studying the binding of I125-labeled thrombin to platelets. The reaction of collagen with platelets will be investigated by studying the kinetics of the collagen-induced secretion of ATP. BIBLIOGRAPHIC REFERENCE: Martin, B. M., Feinman, R. D. and Detwiler, T. C. Platelet Stimulations by Thrombin and other Proteases. Biochemistry 14, 1308 (1975).